The present invention relates to an oxide catalyst, particularly an oxide catalyst suited for producing maleic anhydride by vapor phase oxidation of a hydrocarbon having 4 carbon atoms, such as butane, butene, butadiene or the like, and a process for producing maleic anhydride by vapor phase oxidation of a hydrocarbon having 4 carbon atoms in the presence of the oxide catalyst.
The catalysts comprising tetravalent vanadium and pentavalent phosphorus have been used for production of maleic anhydride by selective oxidation in vapor phase, a hydrocarbon having 4 carbon atoms such as butane, butene, butadiene or the like, especially n-butane which is a saturated hydrocarbon. Among these catalysts, vanadyl pyrophosphate ((VO).sub.2 P.sub.2 O.sub.7) is known as a crystalline composite oxide catalyst having excellent catalytic activities, and there are many literatures relating to this compound (e.g. Chem. Rev. 88. pp. 55-80, 1988, and references cited therein). For the synthesis of divanadyl pyrophosphate, a method comprising heating vanadyl hydrogen phosphate hemihydrate (VOHPO.sub.4.0.5H.sub.2 O) which is a precursor of the compound is generally employed, and it is reported that the precursor, when heated, can be converted into vanadyl pyrophosphate through topotactic transformation.
Many proposals have been made on the preparation of vanadyl hydrogen phosphate hemihydrate used as the precursor. Most of them employ the method for preparing the precursor in an organic solvent (see, for example, Japanese Patent Publication (KOKOKU) Nos. 53-13607 (1978) and 1-50455 (1989)). The method basically comprises reacting a pentavalent vanadium compound with a pentavalent phosphorus compound in a reducing organic solvent while reducing the vanadium compound with the solvent, to obtain a precursor of a crystalline composite oxide of tetravalent vanadium and pentavalent phosphorus.
Proposals have also been made regarding the preparation of a fluid-bed catalyst using vanadyl pyrophosphate precursor prepared in an organic solvent as mentioned above. When crystalline vanadyl pyrophosphate alone is made into a fluid catalyst, the catalyst obtained is very weak in mechanical strength of the catalyst and unusable under the industrial fluidized bed reaction conditions, so studies have been made on the preparation of practical fluid-bed catalysts.
Concerning preparation of a fluid-bed catalyst by use of vanadyl pyrophosphate or its precursor, Japanese Patent Application Laid-Open (KOKAI) No. 57-122944 (1982), for instance, discloses a process which comprises comminuting the catalyst precursor prepared in an organic liquid, introducing the precursor into water to form an aqueous slurry, and spray-drying the slurry. In this process, the catalyst precursor is comminuted so that the substantial portion thereof has an average particle size (diameter) of less than 1 micron, preferably less than 0.5 microns, and ball milling is mentioned as a means for comminuting. As to the catalyst composition, the catalyst precursor synthesized in an organic solvent alone and a mixture of the precursor with silica were actually used. Japanese Patent Application Laid-Open (KOKAI) No. 59-55350 (1984) discloses a process in which the catalyst precursor containing a mixed oxide of vanadium and phosphorus is densified and comminuted to form fluidisable particles and the particles are calcined under the fluidization conditions. In the Examples of these publications, there is shown a method in which the material is comminuted by a ball mill and formed into a water slurry, and the slurry is spray-dried with adding or without adding silica sol. In a Comparative Example, it is shown that when comminuting is carried out by an air mill, the obtained catalyst is low in mechanical strength.
Japanese Patent Application Laid-Open (KOKAI) No. 60-64632 (1985) discloses a process in which a crystalline composite oxide containing tetravalent vanadium with pentavalent phosphorus, obtained by reacting a pentavalent vanadium compound and a pentavalent phosphorus compound in an organic solvent capable of reducing the pentavalent vanadium to the tetravalent state, as the first component; an aqueous solution containing tetravalent vanadium and phosphorus, as the second component; and silica sol used as the third component are mixed to prepare a slurry and the resultant slurry is spray-dried. In the Examples of this publication, the three-component mixed slurry is subjected to wet pulverizing and mixing, then spray dried and calcined to form a fluidisable catalyst.
However, the catalyst obtained according to the conventional production process has not be able to satisfy all of the requirements for an industrial catalyst, as mentioned below.
(i) Excellent mechanical properties such as mechanical strength. PA1 (ii) Excellent reaction performance. PA1 (iii) Low cost and good production reproducibility. PA1 (1) a pentavalent vanadium compound and a pentavalent phosphorus compound are reacted in an organic solvent in the presence of a reducing agent capable of reducing the pentavalent vanadium to the tetravalent state, thereby obtaining the crystalline composite oxide particles containing tetravalent vanadium and pentavalent phosphorus (The first step); PA1 (2) the crystalline composite oxide particles obtained in the first step are dry-pulverized in a high-speed flow (The second step); PA1 (3) the pulverized material obtained in the second step is mixed with an aqueous solution containing tetravalent vanadium and pentavalent phosphorus to form a slurry (The third step); PA1 (4) the slurry obtained in the third step is spray-dried and calcined (The fourth step).
So, improvements on these items have been required.
As a result of the studies by the present inventors, it has been found that an oxide catalyst obtained by the followings can well meet the requirements. First, the crystalline composite oxide particles containing tetravalent vanadium and pentavalent phosphorus are prepared by reaction of a pentavalent vanadium compound and a pentavalent phosphorus compound in an organic solvent in the presence of a reducing agent capable of reducing the pentavalent vanadium to the tetravalent state, then the crystalline composite oxide particles are dry-pulverized in a high-speed gas flow, the pulverized particles are mixed with an aqueous solution containing tetravalent vanadium and pentavalent phosphorus to form a slurry, and the slurry is spray-dried and then calcined. The present invention has been attained on the basis of this finding.